Membrane separation technology is of increasing importance in the separation industry. This membrane technology can be applied in the separation of a range of components of varying molecular weights in gas or liquid phases, including but not limited to nanofiltration, desalination and water treatment. The particular advantage of membrane separation is the absence of heating whereby the use of energy is significantly lower than conventional thermal separation processes (destillation, crystallization, . . . ) (Basic Principles of Membrane Technology, Second Edition, M. Mulder, Kluwer Academic Press, Dordrecht. 564 p).
Membranes are used, for instance, in separation processes as selective barriers that allow certain components to pass, i.e., the permeate, while retaining other compounds, i.e., the retentate. Membranes are used in many applications, for example as inorganic semiconductors, biosensors, heparinized surfaces, facilitated transport membranes utilizing crown ethers and other carriers, targeted drug delivery systems including membrane-bound antigens, catalyst containing membranes, treated surfaces, sharpened resolution chromatographic packing materials, narrow band optical absorbers, and in various water treatments which involve removal of a solute or contaminant for example dialysis, electrolysis, microfiltration, ultrafiltration and reverse osmosis (Membrane technology and applications, R. Baker, John Wiley & Sons, 2004, 538 p).
Although membrane separation processes are widely applied in the filtration of aqueous fluids, they have not been widely applied for the separation of solutes in organic solvents, despite the fact that organic filtrations, such as organic solvent nanofiltration, has many potential applications in industry. This is mainly due to the relatively poor performance and/or stability of the membranes in organic solvents.
Many membranes for aqueous applications are thin film composite (TFC) membranes, which can be made by interfacial polymerisation (IFP). The IFP technique is well known to those skilled in the art (Petersen, R. J. “Composite reverse osmosis and nanofiltration membranes”. J. Membr. Sci, 83, 81-150, 1993). The procedures of U.S. Pat. No. 3,744,642 and U.S. Pat. No. 4,277,244 and U.S. Pat. No. 4,950,404 are illustrative of the fundamental method for preparing thin film composite (TFC) membranes. One of the earliest patents to describe membranes of the type used in the present invention, U.S. Pat. No. 3,744,642 discloses the process of reacting a broad group of aliphatic or carbocyclic primary diamines with aliphatic or carbocyclic diacyl halides on a porous support membrane to form TFC membranes.
In IFP, an aqueous solution of a reactive monomer (often a polyamine (e.g. a diamine)) is first deposited in the pores of a porous support membrane (e.g. a polysulfone ultrafiltration membrane)—this step is also referred to as support membrane impregnation. Then, the porous support membrane loaded with the first monomer is immersed in a water-immiscible (organic) solvent solution containing a second reactive monomer (e.g. a tri- or diacid chloride). The two monomers react at the interface of the two immiscible solvents, until a thin film presents a diffusion barrier and the reaction is completed to form a highly cross-linked thin film layer that remains attached to the support membrane. Since membranes synthesized via this technique usually have a very thin top layer, high solvent permeancies are expected. High flux is often associated with thin membranes, while high selectivity should not be affected by membrane thickness (Koops, G. H. et al. “Selectivity as a Function of Membrane Thickness: Gas Separation and Pervaporation” Journal of Applied Polymer Science, 53, 1639-1651, 1994). Since the first successes reached within this field by Loeb and Sourirajan, extensive research has been performed starting from their reverse osmosis membranes disclosed in U.S. Pat. No. 3,133,132. A subsequent breakthrough was achieved by Cadotte. Inspired by the work of Morgan, who was the first to describe “interfacial polymerization”, Cadotte produced extremely thin films using the knowledge about interfacial polymerization, as claimed in U.S. Pat. No. 4,277,344.
The thin film layer can be from several tens of nanometers to several micrometers thick. The thin film is selective between molecules, and this selective layer can be optimized for solute rejection and solvent flux by controlling the coating conditions and characteristics of the reactive monomers. The (micro)porous support membrane can be selectively chosen for porosity, strength and solvent resistance.
There is a myriad of supports or substrates for membranes. Specific physical and chemical characteristics to be considered when selecting a suitable substrate include: porosity, surface porosity, pore size distribution of surface and bulk, permeability, solvent resistance, hydrophilicity, flexibility and mechanical integrity. Pore size distribution and overall surface porosity of the surface pores are of great importance when preparing a support for IFP.
An example of interfacial polymerization used to prepare TFC membranes are “Nylons”, which belong to a class of polymers referred to as polyamides. One such polyamide is made, for example, by reacting a triacyl chloride, such as trimesoylchloride, with a diamine, such as m-phenylenediamine. The reaction can be carried out at an interface by dissolving the diamine in water and bringing a hexane solution of the triacyl chloride on top of the water phase. The diamine reacts with the triacyl chloride at the interface between these two immiscible solvents, forming a polyamide film at or near the interface which is less permeable to the reactants. Thus, once the film forms, the reaction slows down drastically, leaving a very thin film. In fact, if the film is removed from the interface by mechanical means, fresh film forms almost instantly at the interface, because the reactants are so highly reactive.
Numerous condensation reactions that can be used to interfacially make polymers have been described. Among the products of these condensation reactions are polyamides, polyureas, polyurethanes, polysulfonamides and polyesters (U.S. Pat. No. 4,917,800). Factors affecting the making of continuous, thin interfacial films include temperature, the nature of the solvents and cosolvents, and the concentration and the reactivity of monomers and additives. These polymers however have various disadvantages. Next to poor stability in for instance chlorinated solvents, the polyamides fail to sustain at temperatures higher than 450° C. and outside a pH range of 2-12 (Wang et al. “A polyamide-silica composite prepared by the sol-gel process” Polymer Bulletin, 31, 323-330, 1993). The drawbacks of this traditional IFP product has led to the demand of new, solvent stable membranes with similar performance.
Novel membranes are also needed since there is an interest in operating in organic solvent streams to separate small molecules such as synthetic antibiotics and peptides from organic solutions. In these types of applications, a high permeability is required for economical operation. Polar organic solvents, such as dipolar aprotic solvents, particularly solvents such as N-methyl pyrrolidone (NMP), dimethylacetamide (DMAC), and dimethylsulfoxide (DMSO) are used as solvents or media for chemical reactions to make pharmaceuticals and agrochemicals (for example, pyrethroid insecticides) industry. These powerful solvents will cause severe damage to commonly used polymeric membranes made from polysulfone, polyethersulfone, polyacrylonitrile or polyvinylidene fluoride polymers.
In many applications, it would also be useful for the membrane to operate with aqueous mixtures of solvents or with both aqueous solutions and solvent based solutions in series. For such uses, hydrophobic membranes are not useful as they have very low permeabilities for aqueous solutions.
This had led to a large demand of new, solvent stable membranes. It is an objective of the present invention to provide a highly efficient novel route for the production of such membranes.